1. Field of the Invention
The invention relates to electrophotographic document copiers adapted to reproduce transparency originals, and more particularly to automatic adjustment of parameters influencing the output reproduction of such copiers.
2. Description of the Prior Art
In electrophotographic copiers, image contrast, density, and color balance (in color machines) can be adjusted by changing certain process control parameters such as primary voltage V.sub.O, exposure E.sub.O, development station electrode bias voltage V.sub.b, the concentration of toner in the developer mixture, and the image transfer potential.
Control of such parameters is often based on measurements of the density of a toner image in a test patch. U.S. Pat. No. 4,894,685, issued Jan. 16, 1990, discloses a method for setting color process control parameters by forming a plurality of different density test patches in a non-transfer portion of a photoconductor. Process control parameters are set in accordance with the differences between the recorded densities and aim densities. However well this and similar set-up systems work for reflection originals, they are generally inferior when used to set the process control when copying transparencies, such as 35 mm color slides, to produce pictorial quality color opaque copies.
It has been found that the illumination system used to copy opaque originals placed on a platen does not produce sufficient intensity for copying transparencies because light rays from below the platen must pass through the transparency, be reflected from the platen cover, and pass back through the transparency onto the photoconductive surface.
To overcome the need for additional illumination, the prior art has provided a separate light source which is used to project an image from the transparency onto the platen. See for example U.S. Pat. No. 4,027,962, which issued to L. Mailloux on June 7, 1977. However, copiers that have had their process control parameters adjusted for best results when copying reflection originals with one light source may not be optimized for copying transparencies using a different light source. Also, the two light sources may age at different rates, causing very different changes in intensity and color characteristics.